The Aerospike Engine: How This Astonishing Rocket Nozzle Will Help Future Rockets

Rocket engines play a critical role in space travel, and their efficiency heavily relies on the nozzle design. By exerting a force in the opposite direction, the expelled gasses from the nozzle generate thrust that propels the rocket forward. 

Whether it’s an aerospike engine or a converging-diverging nozzle, the nozzle does the same purpose. Its carefully designed shape and geometry accelerate the exhaust flow, ensuring that the expanding gasses are directed rearward in a precise and controlled manner.

Converging-Diverging Nozzles

1. How Converging-Diverging Nozzles Work?

Most rockets employ Converging-Diverging (CD) nozzles, consisting of two sections: a converging section, where the nozzle narrows and accelerates the gasses to sonic speeds, and a diverging section, where the nozzle expands and further accelerates the gasses to supersonic velocities.

This expansion converts the thermal energy of the gasses into kinetic energy, producing a high-velocity exhaust jet that generates thrust in the opposite direction, propelling the rocket forward. 

Converging-Diverging Nozzle diagram

The shape of the CD nozzle determines how the exhaust gasses expand, affecting the engine’s efficiency. The nozzle’s shape is designed to allow gasses to reach a pressure equal to the open stream pressures exactly when it exit the nozzle.

This way, all gasses will leave the nozzle in a parallel stream, and the thrust will be maximal. If the pressure of exiting gasses is higher than the ambient pressures, the gasses will be under-expanded and will expand sideways, resulting in a loss of thrust.

Similarly, if the pressure of exiting gasses is lower than ambient pressure, the gasses will be overexpanded, and the ambient pressure will constrict the exhaust flow. 

The Problem with Conventional Rocket Engine Nozzles

Hence, the optimal shape of the nozzle depends on the ambient pressure.

Here lies the problem, the ambient pressure decreases as the rocket increases in altitude. Since the nozzle’s shape is set to a particular ambient pressure, its performance decreases when the ambient pressure deviates from this ideal range.

The problem with conventional rocket engine nozzle.
Credit: Headed for Space

Rocket staging partially fixes this issue. By staging the rocket, engineers can incorporate multiple nozzles with different shapes to optimize the exhaust at different ambient pressures.

For example, a two-stage rocket can have one nozzle optimized for sea-level pressure that’s discarded after the initial burn and another nozzle optimized for the vacuum of space. 

However, if we would like to use a vehicle that requires no staging, we need a rocket engine with optimal performance throughout the ascent. Let me introduce you to the Aerospike engine. 

How does the Aerospike Engine Work?

Aerospike engines offer a solution to the challenges faced by traditional rocket engines when operating at different altitudes.

Unlike the CD engines that struggle to function efficiently at various pressure levels, aerospike engines utilize the surrounding ambient air pressure to shape the walls that contain the exhaust flow, ensuring optimal conditions are maintained at all altitudes.

Diagram of how aerospike engines work
Credit: Stack-Exchange

In an aerospike engine, the throat of the combustion chamber aims the exhaust gas at the “spike” in the nozzle, which is essentially the shape of an inverted bell.

At sea level, the higher ambient pressure squeezes the lower-pressure exhaust against the nozzle wall, preventing flow separation. 

As the vehicle ascends, the decreasing ambient pressure decreases the pressure against the exhaust. This allows the nozzle’s virtual outer wall to expand and adjust its expansion ratio with the altitude.

The length of the spike in the aerospike engine determines this expansion ratio.

Many aerospike designs feature a flat base or truncated spike, which uses the turbine exhaust and a heat exchanger to create a pressure zone that boosts thrust at higher altitudes, enhancing the engine’s advantages further.

The Different Types of Aerospike Engines

1. Linear Aerospike Engine

linear aerospike engine mockup
Credit: Martian Days

When the inverted bell is flattened and laid out, it is called a linear aerospike. The linear aerospike incorporates a series of small combustion chambers, referred to as the ramp, situated along the unwrapped bell.

These chambers propel hot gasses along the ramp’s outer surface, generating thrust along its entire length.

In the aerospike engine, the ramp functions as the inner wall of the virtual bell nozzle, while the ambient pressure acts as the “invisible” outer wall. Combustion gasses accelerate between the inner wall (ramp) and the outer wall (ambient pressure) to produce thrust.

2. Toroidal Aerospike Engine

torodial aerospike engine nozzle
Credit: Wiki-visually

A Toroidal Aerospike Engine operates on a similar principle to other aerospike engines, but it has a specific toroidal (doughnut-shaped) geometry for its exhaust nozzle.

The engine consists of a combustion chamber with multiple nozzles arranged in a ring around the central axis, forming a toroidal shape.

At lower altitudes, the ambient air pressure compresses the exhaust gasses against the toroidal shape, creating a recirculation zone at the base of the nozzle.

This recirculation zone effectively balances the pressure in that region, reducing thrust loss at low altitudes.

As the vehicle climbs to higher altitudes, the ambient air pressure decreases, leading to a decrease in the pressure holding the exhaust against the toroidal shape.

As a result, the exhaust gasses expand outward, generating additional thrust as altitude increases.

3. Annular Aerospike Engine

An Annular Aerospike Engine consists of a central spike surrounded by a ring-shaped exhaust channel. 

At lower altitudes, the surrounding air pressure compresses the exhaust gasses against the ring-shaped exhaust channel, forming a circular zone around the central spike.

This zone helps maintain pressure balance, minimizing thrust loss at low altitudes.

As the vehicle ascends to higher altitudes, the ambient air pressure decreases, causing the exhaust gasses to expand outward as they are no longer constrained by the ring-shaped exhaust channel. This expansion generates extra thrust, enhancing performance as altitude increases.

The Disadvantages of Aerospike Engine

Despite the advantages of aerospike engines, two main challenges hinder their widespread adoption in rocket design: cooling the engine and weight. Cooling an aerospike engine proves challenging due to the substantial surface area that needs cooling.

For example, in a toroidal aerospike engine, the combustion chamber basically houses a giant plug in the center, significantly increasing the surface area that requires cooling.

Furthermore, the thrust-to-weight ratio of aerospike engines is often lower compared to CD nozzles because of the larger surface area of the combustion chamber, additional plumbing, and supporting structures adding to the weight.

The toroidal or linear shape of the combustion chamber in aerospike engines requires more material and structure to achieve the advantage of a self-regulating nozzle shape.

The presence of multiple combustion chambers necessitates additional plumbing to supply propellants to these chambers effectively. 

Moreover, supporting structures are required to hold the aerospike engine in place within the rocket’s structure, given the large size and unique attachment points of the engine. 

Conclusion

The concept of aerospikes is undeniably fascinating, luring even the most astute aerospace engineers and presenting a perfect fit for Single-Stage-To-Orbit (SSTO) concepts.

However, despite their appeal, these engines come with significant challenges that have hindered their widespread adoption.

If aerospike engines offered a clear performance advantage over traditional rocket engines, we would likely see more companies pursuing their development.

Yet, in practice, aerospike engines tend to end up on par with other engines, with their added disadvantages counterbalancing the advantage of efficient exhaust in all altitudes.

In conclusion, the appeal of aerospikes is undeniable, but their challenges and practicality have relegated them to a niche status in the aerospace industry.

As research and development progress, perhaps we will witness advances that enhance the viability of aerospike engines. Ultimately, for now, the Converging-Diverging nozzle remains the prevailing choice for most rocket propulsion applications.

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